1. Field of the Invention
The invention concerns a composite film for packaging purposes with good permeability barrier effect for water vapor and gases after sterilization in a water bath or in water vapor at temperatures of more than 90xc2x0 C., where the composite film also has as a substantial constituent a barrier layer with SiOx which is produced by the vaporization of inorganic materials. Also within the framework of this invention lies a process for the production of the composite film and its usage.
2. Background
In a recognized method of prolonging the durability of perishable products such as foodstuffs, the products are sterilized in a packaged state. To achieve this the filling material is heated briefly in its sealed packaging by autoclaving in hot water or water vapor at temperatures of up to 130xc2x0 C.
The known transparent composite films used today for packaging foodstuffs often lack sufficient barrier properties to water vapor, oxygen and aromas after sterilization treatment. Examples are ethylvinyl alcohols (EVOH) and copolymers of EVOH and polyethylene (PE), the barrier properties of which deteriorate especially in very moist conditions, resulting in a milky appearance. Better barrier properties are achieved by coating a silicon monoxide-coated film of polyethylene terephthalate, but when heated at high temperatures these films show not only a yellowish discoloration but also a decrease in barrier properties.
The invention is therefore based on the task of creating a composite film of the type described initially which shows improved barrier properties with regard to water vapor, oxygen and aromas after sterilization treatment in comparison with state of the art transparent film laminates.
The solution of the task according to the invention leads to the composite film comprising a film of polyethylene terephthalate (PET) with a coextruded layer of polyethylene-2,6-naphthalate (PEN) on at least one side, and the film on at least one of the PEN-coated sides has a 10 to 200 nm thick ceramic layer of SiOx, where x is a number between 0.9 and 2, produced by the simultaneous vaporization of silicon dioxide (SiO2) and metallic silicon in a vacuum.
The term PEN is used below to mean not only the pure polymer but also a mixture of polymers consisting of at least 60 w. % ethylene-2,6-naphthalate units and up to 40 w. % ethylene terephthalate units and or units of cycloaliphatic or aromatic diols and or dicarbonic acids.